U.S. patent number 5,203,026 [Application Number 07/673,304] was granted by the patent office on 1993-04-13 for method of selecting the most suitable receiver antenna from two or more receiver antennas.
This patent grant is currently assigned to Telefonaktiebolaget L M Ericsson. Invention is credited to Bjorn O. P. Ekelund.
United States Patent |
5,203,026 |
Ekelund |
April 13, 1993 |
Method of selecting the most suitable receiver antenna from two or
more receiver antennas
Abstract
The invention relates to the selection of an antenna in a
receiver of a mobile radio station which forms part of a
time-multiplexed radio communication system. The signal strength of
each antenna is measured in each time slot which immediately
precedes the time slot intended for the station concerned, and the
antenna selection is effected in dependence on the measured signal
strengths. In order to eliminate the risk of unsuitable antenna
selection as a result of momentary falls in signal strength, the
signal strength for each antenna is measured on several occasions
during the time slot in which the measurements take place. The
signal-strength values for each antenna are thereafter combined
linearly, so as to form a suitability measurement. The antenna
having the highest suitability measurement is selected for the next
arriving time slot.
Inventors: |
Ekelund; Bjorn O. P. (Lund,
SE) |
Assignee: |
Telefonaktiebolaget L M
Ericsson (Stockholm, SE)
|
Family
ID: |
20379149 |
Appl.
No.: |
07/673,304 |
Filed: |
March 22, 1991 |
Foreign Application Priority Data
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Apr 10, 1990 [SE] |
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9001311-1 |
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Current U.S.
Class: |
455/134; 370/334;
375/347; 455/277.1 |
Current CPC
Class: |
H04B
7/082 (20130101) |
Current International
Class: |
H04B
7/08 (20060101); H04B 017/02 () |
Field of
Search: |
;455/134,273,275,277.1
;375/40,100 ;370/95.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Keough; Timothy H.
Attorney, Agent or Firm: Burns, Doane, Swecker &
Mathis
Claims
What is claimed is:
1. In a time-multiplexed radio communication system having a mobile
radio station receiver, the system including an allocated time slot
during which the receiver is intended to communicate, a method of
selecting, from two or more antennas in the receiver, a most
suitable antenna for communication during the allocated time slot,
comprising the steps of:
for each of the antennas, making a plurality of signal strength
measurements during a time interval in a time slot which precedes
the allocated time slot, wherein the duration of the time interval
is short in relation to the duration of the time slot;
for each of the antennas, linearly combining the plurality of
signal strength measurement values to form a suitability
measurement that is indicative of the antenna's suitability for use
in the receiver during communication in the allocated time slot,
wherein the step of linearly combining includes the step of
multiplying each of the plurality of signal strength measurement
values by a coefficient that is a function of a time derivative of
the signal strength measurement values; and
selecting an antenna for use in the receiver during communication
in the allocated time slot by choosing an antenna having a highest
suitability measurement.
2. In a time-multiplexed radio communication system having a mobile
radio station receiver, the system including an allocated time slot
during which the receiver is intended to communicate, an apparatus
for selecting, from two or more antennas in the receiver, a most
suitable antenna for communication during the allocated time slot,
comprising:
a plurality of generating means for generating a suitability
measurement of an antenna, the suitability measurement being
indicative of the antenna's suitability for use in the receiver
during communication in the allocated time slot, each generating
means coupled to receive a plurality of signal strength measurement
values made from an antenna signal during a time interval in a time
slot which precedes the allocated time slot, wherein the duration
of the time interval is short in relation to the duration of the
time slot, and wherein the generating means linearly combines the
plurality of signal strength measurement values to form the
suitability measurement, and wherein further the generating means
multiplies each of the pluraliity of signal strength measurement
values by a coefficient that is a function of a time derivative of
the signal strength measurement values in order to produce the
suitability measurement; and
comparison means for generating an antenna selection signal which
is indicative of an antenna that should be used in the receiver
during communication in the allocated time slot, the comparison
means being coupled to receive the suitability measurements from
the plurality of generating means, and wherein the comparison means
generates the antenna selection signal by determining which antenna
corresponds to a highest suitability measurement.
Description
TECHNICAL FIELD
The present invention relates to a method of selecting the most
suitable antenna of two or more antennas in a receiver of a mobile
radio station included in a time-multiplexed radio communication
system for each time slot intended for the station concerned,
wherein the received signal strength for each antenna is measured
during a time slot which immediately precedes the time slot
intended for the station concerned, and wherein the selection of
the antenna is effected in dependence on these signal
strengths.
BACKGROUND ART
In a radio communication system in which at least one party of an
established telephone connection is mobile, the transmitted radio
signals are subjected to so-called Rayleigh fading. This means that
the radio wave transmitted directly from a transmitter to a
receiver will interfere with a large number of reflected waves, for
instance waves that have been reflected by buildings, hills and
vehicles, for instance. One characteristic feature of such fading
is that the strength of the signal received can be very low over
short periods of time, although still have a high mean value.
Similarly, the strength of the signals received may be very low
when communicating between two stationary stations. Consequently, a
connection may be interrupted or broken when a mobile station, for
instance a vehicular mobile telephone stops at a location in which
the received signal strength is low.
When the wavelength is short, for instance at the frequency of 900
MHz, two antennas located only a short distance from one another
may exhibit totally different fading patterns. For instance, the
received signal strength in the one antenna may be high while, at
the same time, the signal strength in the other antenna is low,
whereas the conditions may be the reverse only a short time
thereafter. In so-called diversity reception, there is used at
least two receiver antennas and the signals from these antennas are
added in some suitable manner, or alternatively only the strongest
signal at that moment in time is chosen. The procedure of adding
several signals in a manner such that the signals will coact
mutually and not extinguish one another is complicated,
however.
EP, A2, 0 318 665 teaches two methods of selecting one of two
receiver antennas continuously. According to one method, the
strength of the signal received on the antenna used during a time
slot in which transmission is effected to the receiver concerned is
measured and stored. During a following time slot, when
transmission is effected to another receiver, the strength of the
signal for the other antenna is measured, and the measured value is
then compared with the stored value of the antenna previously used.
The antenna to be used during the next time slot, where
transmission is to be effected to the receiver concerned, is then
chosen on the basis of the highest value obtained.
According to another method, the signal strengths of both antennas
are measured during a time slot in which transmission is effected
to another receiver, whereafter the antenna having the highest
signal strength is used during the time slot in which transmission
shall be effected to the receiver concerned.
When antenna selection is effected with the aid of known methods,
there is a danger that the signal strength of one antenna will be
measured at precisely that moment when signal strength is very low.
There is therefore a risk that a less suitable antenna will be
selected, i.e. the antenna selected will not be the antenna which
would provide the highest signal strength during the time slot in
which transmission shall take place to the receiver concerned.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide a method of the
kind defined in the introduction which will reduce the risk of
selecting a less suitable antenna manifest with known methods. This
is achieved by measuring the signal strengths of each respective
antenna at different moments during the time slot which immediately
precedes the time slot intended for the station concerned, and by
combining the values of the measured signal strengths linearly in
an appropriate manner. In this way, there is formed for each
antenna a suitability measurement which, for instance, is
proportional to the mean value of the signal strength or which is
more dependent on the signal strength values at the end of a
measuring period than at the beginning of a measuring period during
which the signal strength is measured several times, or which is
dependent on both the magnitude of the signal strength and its time
derivation.
The characteristic features of the invention are set forth in the
following claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail with reference
to the accompanying drawings, in which
FIGS. 1a and 1b show, respectively, a receiver connected to two
antennas located at a relatively short distance apart, and a
diagram illustrating an example of variations in time of the
strengths of signals received by the two separate antennas;
FIG. 2 illustrates an example of an arrangement for carrying out
the method according to the invention; and
FIG. 3 is a diagram which illustrates another example of the time
variations of two received signal strengths.
BEST MODES OF CARRYING OUT THE INVENTION
FIGS. 1a and 1b are diagrams that exemplify how the strengths of
the signals received by a receiver 3 from two separate antennas can
vary with time. The antennas are located at a relatively short
distance apart and may, for instance, be mounted on a moving
vehicle. The signal strength is designated s(t) in the diagram
shown in FIG. 1b. The full line curve, s1, illustrates the signal
strength on, one of the antennas, in this instance the antenna 1
shown in FIG. 1a, and the broken-line curve s2, illustrates the
signal strength on the other of said antennas, in this case the
antenna 2, shown in FIG. 1a. The reference signs A1 and A2 on the
time axis indicate which of the antennas 1 and 2 has the highest
received signal strength at that moment in time. As is evident from
the curves, the received signal strength can be relatively low over
short time periods. Although not shown in FIG. 1b, in the case of a
time multiplex mobile telephony system the time distance between
such pronounced decreases in signal strength are of the same order
as the duration of a time slot.
As before mentioned, there is a danger that the wrong antenna will
be selected because the strength of the signal received by the
other antenna was perhaps measured during a brief fall in signal
strength. It is assumed here that the signal strengths in the two
antennas are measured in a time slot in which there are received
signals intended for another station, and that this time slot
immediately precedes a time slot in which information is
transmitted to the receiver concerned.
According to the present invention, the risk of the wrong antenna
being selected is reduced by measuring the signal strength of each
of the two antennas on several occasions during a measuring period
which falls in the time slot in which the measurements are taken.
For instance, a suitability measurement in the form of a mean value
of the signal strength of each antenna can thereafter be calculated
and these mean values compared with one another. The antenna chosen
is the antenna which has the largest calculated mean value. When
forming mean values, the measuring period during which several
measurements of the various signal strengths are taken should not
be appreciably shorter than the duration of a time slot, otherwise
it would be possible to measure all of the signal strength values
of one of said antennas during one of the brief periods in which
signal strength is pronouncedly decreased, therewith giving a very
low and misleading mean value of the signal strength for this
antenna.
In order to prevent antenna selection being unduly influenced by
noise, at least ten signal strength measurements should be made
with each antenna. A suitable value of the duration of a measuring
period is about 30% of the duration of a time slot.
FIG. 2 is a block schematic which illustrates an example of an
arrangement for carrying out the inventive method. The measured
values of the signal strengths from two receiver antennas, antenna
1 and antenna 2, are referenced s1.sub.i and s2.sub.j respectively.
The values are delivered to two analog-digital converters AD1, AD2
and are then transmitted, in digital form, to two storage devices
Ml, M2. The signal-strength values are transmitted from the storage
devices Ml, M2 to two processing means B1, B2, in which a
suitability measurement for each antenna is calculated, by
processing these values in a predetermined manner. As before
mentioned, the mean value of the signal-strength values can be
calculated, for instance. Naturally, the measurement values may
alternatively be simply summated for each antenna per se, thereby
obtaining for each antenna a suitability measurement which is
proportional to the mean value of the signal-strength values,
without disclosing the actual mean value itself. The two quality
measurements are finally compared in a comparison means J,
wherewith an antenna is selected o the basis of the highest
suitability measurement.
It is also conceivable to weight the measurement values in a
suitable manner, i.e. by multiplying each of said values with a
respective coefficient prior, to adding said values together. For
instance, those measurement values taken at later times can be
multiplied with higher multipliers than the multipliers used to
multiply earlier measurement values.
It can be mentioned that the signal strength of the antenna 1 is
preferably measured at slightly different time points in relation
to the signal strength of the antenna 2, therewith reducing the
need of receiver devices.
FIG. 3 is a second diagram showing examples of the strengths of
signals received by two separate antennas. Similar to the FIG. 1
illustration, the signal strengths are referenced s1 and s2. The
reference sign T signifies a time slot during which information
shall be transmitted to a station for which one of two receiver
antennas shall be selected. The decision as to which of the antenna
shall be selected is made at the end of a measuring period M which
lies at the end of a time slot which immediately precedes the time
slot T and during which information is transmitted to another
station. The signal strengths s1 and s2 are each measured an
appropriate number of times during the measuring period M. As
evident from the FIG., the signal strength s2 has a higher mean
value than the signal strength s1 during the measuring period M.
However, it is more suitable to select the antenna 1 having the
signal strength s1 than the antenna 2, since the signal strength s1
is higher than the signal strength s2 over the major part of the
time slot T. In this case, the measuring period M is short in
relation to the duration of a time slot. A suitable value is about
5% of the duration of a time slot. It is therefore appropriate that
the suitability measurements calculated in the processing means B1,
B2 in FIG. 2 are, in this case, dependent on both signal strength
and its time derivation.
A simple example of how a suitability measurement can be calculated
in dependence on signal strength and time derivation can be
described in the following way. Assume that only three
signal-strength samples are taken for each antenna and that these
samples are referenced x, y and z. A suitability measurement L can
then be calculated in accordance with the formula L=x+y+z+a
(z-y)+b(y-x), where a and b are appropriately selected constants.
This expression can be simplified to (1-b)x+(1-a+b)y+(1+a) z, which
is a linear combination of x, y and z. The three terms x, y and z
at the beginning of the formula may, of course, each be multiplied
by a constant which is analogous with what was mentioned earlier.
This will only change the coefficients in the linear combination to
be carried out in each of the processing means B1, B2. In practice,
the signal strength should also be measured at least about ten
times with each antenna, in order to prevent the choice of antenna
being influenced unduly by noise. A more realistic example of how a
suitability measurement which is dependent on signal strength and
the derivative can be calculated is described in the following way.
When considering a plurality of signal-strength samples R.sub.i
=R.sub.1 R.sub.2 R.sub.3 ... R.sub.N taken at time points t.sub.i
=t.sub.1 t.sub.2 t.sub.3 ... t.sub.N, the samples can be imagined
as being distributed along an imaginary straight line. The purpose
of the line is to form an approximation of signal strength as a
function of time if the signal strength had not been influenced by
noise. The equation of the line is designated R =A +B.multidot.t,
where A and B are constants. These can be evaluated by means of the
least square method in the following way: ##EQU1## and where all
summations proceed from i=1 to i=N.
It can be found that when a suitability measurement L is calculated
as L=A+K.multidot.B, where K is a constant, this suitability
measurement will constitute a linear combination of the
signalstrength samples R.sub.i. This is shown below in the
particular case when N=4, i.e. with only four signal-strength
samples, and t.sub.1 =0, t.sub.2 =T, t.sub.3 =2T, t.sub.4 =3T. This
gives the following values: ##EQU2## The numerator in the
expression for B becomes: ##EQU3## The denominator in the
expression for B becomes: ##EQU4## Therefore, when the numerator is
divided by the denominator, there is obtained: ##EQU5## When the
value for B is inserted in the expression for A, there is obtained:
##EQU6## The suitability measurement L=A+K.multidot.B then becomes:
##EQU7## This expression is a linear combination of the
signal-strength samples R.sub.1 -R.sub.4. The illustrated example
can be generalized for any selected number of signal-strength
samples.
In the equation for the imaginary straight line R=A+Bxt, A
represents the initial value of the line which, when the line is
short, corresponds approximately to the level of the signal
strength, whereas B represents the slope of the line. The constant
K in the suitability measurement L=A+K.multidot.B may suitably have
the value of about 5-10, which implies that the significance of the
slope of the line is considered to be 5 to 10 times greater than
the significance of the signal-strength level. This should be
appropriate when the measuring period M is considerably shorter
than a time slot.
As will be understood, the aforedescribed inventive method can also
be applied in stations which include more than two antennas,
wherein a suitability measurement is calculated for each antenna,
as in the case with solely two antennas.
* * * * *